384 Merrill — Metamorphic Strata of S. E. New York. 



described by him as being the oldest formation exposed in that 

 state, and almost entirely composed of " hornblendic granite " : 

 second, an Iron-bearing Group, resting conformably on the pre- 

 ceding, well stratified, and comprising bedded granulites and 

 magnetite deposits, above the latter being crystalline limestones 

 containing magnesian silicates in various stages of alteration : 

 third and uppermost, a " Schistose Group," composed of mica- 

 ceous gneisses, of mica schists sometimes containing graphite, 

 and hornblendic and pyroxenic schists of varied composition. 

 The basal Paleozoic quartzite has not yet been found to rest 

 on strata of the third group, and the relative age of the latter 

 remains in doubt. 



In the study of the metamorphic rocks of the New Jersey 

 Highlands and in a preliminary examination of the Hudson 

 River section, the writer was associated with Dr. Britton and 

 has since made a more extended study of the latter. The facts 

 there observed corroborate the general results of Dr. Britton's 

 stratigraphical work in New Jersey. 



The basal member of the pre-Cambrian terrane of southeast- 

 ern New York and New Jersey, is a granitoid hornblende- 

 gneiss consisting chiefly of quartz, orthoclase, plagioclase and 

 hornblende. Magnetite and zircon sometimes occur as acces- 

 sories. As stated by Dr. Britton the rock is devoid of bedding 

 planes send has only a parallel arrangement of its minerals to 

 denote macroscopically its sedimentary origin. Its microscopic 

 structure, however, also suggests its detrital and metamorphic 

 character. 



This rock is not a granite although usually called by that 

 name. In addition to the parallel arrangement of its minerals 

 which would differentiate it from the normal granites, its struc- 

 ture shows that it never attained that condition of molecular 

 freedom which would be necessary to enable its mineral com- 

 ponents to assume a crystalline form. The quartzes arid feld- 

 spars occur in irregular masses interlocking at their margins 

 and in no case show crystalline boundaries. The hornblende, 

 which is allotriomorphic and fills the irregular interstices be- 

 tween the other mineral particles, has probably been developed 

 in the process of metamorphism from aluminous mud contain- 

 ing iron, lime and magnesia. As in rocks which have been 

 developed from the plastic or fluid state the magnesia-iroia 

 silicates according to Rosenbusch, are idiomorphic with respect 

 to the feldspars and the latter are idiomorphic with respect to 

 the quartzes, the structure of the rock in question seems to 

 demonstrate that its component minerals are not developed 

 from a magma but are merely the fragmental particles of a sedi- 

 mentary rock and that during metamorphism only a slight de- 

 gree of molecular freedom was attained by them. 



